ITMI20090092A1 - EQUIPMENT, SYSTEM AND PROCEDURE FOR THE DUCT OF SUBMARINE PIPES - Google Patents

Description

"EQUIPMENT, SYSTEM AND PROCEDURE FOR TOWING SUBMARINE PIPES"

The present invention relates to an apparatus, a system and a process for towing subsea pipelines.

More specifically, the invention relates to techniques for laying pipelines in shallow or very deep waters, in areas subject to currents, by towing said pipelines near the seabed (off-bottom tow).

In the oil field, the use of methods and related equipment for the construction of sections of marine pipelines by means of the towing of prefabricated "strings" of pipe from the production site to the final site is known. Among the various possible methods, the so-called "off-bottom tow" is to be considered among the most used. The pipeline is towed by making it travel just above the seabed.

Although this methodology is used it has some drawbacks including the fact that, in the presence of transverse currents, there may be a displacement of the pipeline off course.

In the patent application WO2006 / 045357 (U. Giovannini et al.) Some solutions are mentioned in this regard and specifically claimed a technique which is based on a geometric control of the configuration of the pipeline with respect to the seabed and not to the depth through the use of a series of equipment, connected to the pipeline itself, substantially containing floats, of which at least one part with variable thrust or partially variable thrust which, placed in a predetermined position and at predetermined intervals, impart to the pipeline a graduated thrust according to the distance from the bottom in so as to obtain a "festoon" configuration of the pipeline, that is, with suspended pipeline sections, in correspondence with the floats, alternating with supporting sections. The same off-bottom shooting technique has been addressed, as recalled in the cited PCT application, in US-4138853 (J.E. Lamy), which proposes, to counteract the action of the transverse current, the use of a device, according to various embodiments, with support elements deformable on the vertical plane but rigid transversely.

The solutions proposed by Giovannini and Lamy are not without drawbacks. In fact, if the former carries out a control of the thrust of the floats with respect to the seabed, it is also true that, in order to maintain the position en route, it relies on a sufficient friction force between the tube and the seabed to counteract the force of the transverse current. This approach is not without risks due to the randomness, by its very nature, of friction. On the other hand, the Lamy solution has, in a complementary way, an approach aimed at counteracting the force of the current but does not contemplate a specific control of the residual weight of the pipeline, depending on its distance from the seabed, with the risk that varying the operating conditions, it is not possible to effectively control the structure of the pipeline during the shooting, nor the residual weight force that its support organs discharge on the seabed, with the risk of encountering considerable resistance to advancement. Furthermore, both techniques do not contemplate any way or device to bring the towed pipe back into position, in the event that it has in any case been diverted by the current (for example during adverse weather and sea events of particular intensity) or in cases where it is necessary to change the route of the towed string (for example to give it a curvature).

In patent application WO2008 / 017463 (C. Cocca) these drawbacks are solved by means of a device and a method for towing subsea pipelines in which the vertical and horizontal attitude control functions are integrated, by means of the of floats with variable or partially variable thrust and guide elements which interacting with the seabed with a partial penetration of a substantially vertical element whose plane is able to rotate on a vertical axis, for example a blade or a disk, are able to maintain the towing direction by counteracting the force of the current or to change the direction.

The solution described by Cocca solves both the problems of determining the thrust of the floats, having only to determine the variation margins of the thrust and not a precise value of the latter, and the uncertainties deriving from the sliding of the pipeline on the bottom, which now it travels parallel to the bottom and not in contact with it.

However, a problem remains, relating to the two previous technologies, which consists in the fact that in the presence of extreme boundary conditions (irregularity of the seabed, significant wave motion, etc.) the behavior of the string could be unstable or in any case not very controllable.

We have found that by using equipment consisting of fixed thrust floats together with equipment consisting of particular floats with variable or partially variable thrust, we are able to solve or significantly reduce this problem. The invention introduces a further way to control the vertical position of the pipeline, again with the aim of reducing the force required for towing. The piping will maintain a â € œfestoonâ € configuration being lightened by a series of floats placed on it at a convenient distance from each other. Towing can take place by means of tugs or suitable traction means (winches) by connecting a towing cable to the head of the pipeline.

The floats that must be used are of two types:

fixed thrust float or â € œpassiveâ € float

variable or partially variable thrust float or â € œactiveâ € float. Both â € œactiveâ € and â € œpassiveâ € floats will always be of the â € œair bellâ € type consisting of prismatic containers capable of being filled or emptied of water thanks to the emission or discharge of air therein. introduced, the container remaining open to the surrounding environment through an opening on its lower surface so that it remains in any case at ambient pressure. The air supply takes place through a special duct common to all the floats (common rail) which is in turn connected to one or more compressors or, in case of need, also individually. The pipeline will be lightened by a combination of these two floats in variable number and arrangement according to the specific use. In principle, the passive floats serve to significantly reduce the weight of the pipeline in water up to the desired total residual weight value, while the active floats serve to compensate for the uncertainties relating to the actual weight of the pipeline with respect to the nominal conditions and guarantee stability. of the conduct itself during shooting operations.

The â € œpassiveâ € floats used are those already known in the field.

With the help of fig. 1 we give a brief description.

The â € œpassiveâ € float is a prismatic container with two mandatory and two optional steps:

a water passage - bidirectional inlet / outlet hole (WP)

an air passage - one-way inlet - consisting of a valve (AP)

a possible passage of control of the â € œtoo fullâ € â € “bidirectional inlet / outlet consisting of a hole (TP)

a possible flooding passage of the float - bidirectional inlet / outlet - consisting of a valve (VA)

The WP passage is a large opening hole placed on the lower surface of the float to make the air bell. It allows the entry and exit of water for the flooding / emptying of the float without offering significant resistance as the WP hole is â € œgreat enoughâ € to not induce overpressure inside the float when the water passes.

The hole WP should be calibrated in such a way that the outflow of the water from the hole WP towards the outside of the float is slowed down, which occurs in the event of a sudden upward motion of the float. The upward motion of the float in fact generates an expansion of the air contained therein due to the lower external environmental pressure (lower depth or head). Basically, by throttling the hole WP the flow of water expelled by the float is limited, hence the expansion of the air and that is, in other words, the increase in thrust of the float. It is thus possible to limit the ascending speed of the float from above to a desired level depending on the diameter of the hole WP and the boundary conditions (depth, float dimensions, etc.), said speed being calculated with the known principles of hydrodynamics applied to the float and to the outflow from the WP hole. In this case the float will have to be sized to withstand the overpressures due to this throttling of the passage: the designer will have to choose the best compromise regarding the strength of the float (structural requirements) and functional requirements.

AP is a passage for the entry of air (made with a nozzle) to displace the water inside the float. The filling percentage of the float is predefined at a level L1 and controlled by a valve that closes the introduction of air inside the float when the desired level L1 is reached. The valve can be controlled electronically (by measuring level L1) or more simply mechanically by means of a float device (â € œflushingâ € type) active at level L1. The valve, continuously fed upstream, automatically introduces air in the event that the float is flooded and in the event of a change in the free surface of the water with respect to level L1 due to air loss or air compression due to a change in height (lowering) of the float. The valve is typically of the open / closed two-state all-or-nothing type. TP is a passage placed on one of the walls of the float that connects with the external environment. It consists of a hole (or a series of holes at various selectively pluggable levels) which defines a predetermined level L2 of maximum emptying: the air inserted in the float once this level L2 has been reached escapes from the hole without further increasing the thrust. It is a device to prevent the float from pushing beyond what is necessary due to a failure of the AP valve or due to the expansion of the air inside the float to raise the float itself. In this way, among other things, the same float can be used in projects with different design thrust without risk.

VA is a passage controlled by a valve (manual) to optionally flood the float quickly allowing air to exit / water to enter. It is placed on the top of the float.

The equipment constituted by the â € œactiveâ € float is new and constitutes a first object of the invention.

This equipment, used for towing underwater pipelines, includes:

a float (G), with a prismatic shape with a horizontal or vertical axis, with variable or partially variable thrust, whose lower base is at least partially open through an opening or hole (WP) to operate as a â € œ bell of airâ €, having:

at least one inlet (AP), consisting of a valve, through which air is supplied;

at least one outlet (RV), consisting of a valve, through which the air is discharged;

an actuator for controlling the valve for supplying air;

an actuator for controlling the valve to discharge air;

means for connecting said apparatus to the pipeline;

means for transferring air to said float;

means for operating the actuator to control the valve (AP) to supply air according to a predefined level in the float;

means for operating the actuator to control the valve (RV) to discharge air according to the distance of the float itself from the bottom.

With the help of fig. 2 we give a brief description.

Said â € œactiveâ € float is a prismatic container with two mandatory and two optional steps:

â € ¢ a water passage â € “bidirectional inlet / outlet (hole) (WP)

â € ¢ an air passage â € “one-way inlet- consisting of a valve (AP)

â € ¢ a possible control passage of the â € œtoo fullâ € â € “bidirectional entrance / exit consisting of at least one hole (TP)

â € ¢ a possible flooding passage of the float - bidirectional inlet / outlet - consisting of a valve (VA)

â € ¢ an air passage â € “one-way outlet â €“ consisting of a valve (RV).

The passage RV is a passage for the exit of the air placed at a desired level L3 in order to be able to evacuate the air inside the float leaving a residual volume that guarantees a residual thrust. L3 can in any case correspond to the top of the float which, in this case, has a completely variable thrust. RV consists of a valve which is operated according to the distance q (altitude) of the float from the bottom F. In particular, upon reaching (exceeding) a predetermined altitude threshold q the RV valve is opened by discharging into the environment surrounding air inside the float which therefore loses thrust. The measurement of the height and the opening of the valve can be carried out with various devices (e.g. acoustic measurement of the height and electrically piloted valve) or simply with a purely mechanical device consisting of a dangling chain with the end part in contact with the seabed and connected to a counterweight device which operates the valve. The valve can be of the â € œall or nothingâ € type with two open / closed states or of the proportional type, with variable light from 0 to 100% of the maximum passage according to the value of the threshold being exceeded. In a further embodiment of the active float, in order to improve the response of the system, the WP hole is calibrated as already described for the passive float in such a way that the outflow of the water, in the event of a sudden upward motion of the float and therefore of expansion of the air contained therein, is slowed down. In this case the float must be sized to resist overpressure.

The control logic of the float valves taken individually is, under normal operating conditions, given in the following table:

Float Valve The float The float goes down and rises

Passive AP Remains closed Opens * to restore level L1 reduced due to the effect of air compression TP Al (not influencing)

achievement

of level L2

discharges the air

in excess

WP Allows Allows the flow and evacuation of water without overpressure pushed water

from expansion

of the air

Active AP Remains closed Opens * for

up to restore the level at intervention L1 reduced for RV and therefore the effect of

opens * when the air compression level L1

is lost

TP Al (not influential)

achievement

of level L2

discharges the air

in excess

RV On exceeding A level below the threshold level, it remains closed or the threshold opens * closes * if previously open (float reversal) * The opening and closing of the valves are conditioned by the presence of a â € œband deadâ € which, around the intervention threshold value, does not cause them to change status. Deadband ensures better control stability.

There is a temporary condition for which the AP and RV valves can be 5 simultaneously open, with simultaneous introduction of air from AP and evacuation from RV. This happens when the altitude q is over the threshold and due to the evacuation of the air from the RV level L1 is lost.

The second object of the present invention is the system for towing subsea pipelines which substantially comprises:

firing means connected to said duct by means of a firing head for firing the same;

â € ¢ a series of equipment, connected to the pipeline itself, each consisting substantially of a float with variable or partially variable thrust a float (G), with a prismatic shape with horizontal or vertical axis, with variable or partially variable thrust, whose base lower is at least partially open through an opening or hole (WP) to operate as an â € œair bellâ €, having:

â € ¢ at least one inlet (AP), consisting of a valve, through which air is supplied;

â € ¢ at least one outlet (RV), consisting of a valve, through which the air is discharged;

â € ¢ a possible control passage of the â € œtoo fullâ €, bidirectional inlet / outlet, consisting of a hole (TP); â € ¢ a possible flooding passage of the float, bidirectional inlet / outlet, consisting of a valve (VA);

â € ¢ an actuator to control the valve to feed air operated by means according to a predefined level in the float; â € ¢ an actuator to control the valve to discharge air operated by means according to the distance of the float itself from the bottom;

â € ¢ means for supplying compressed air to the floats;

â € ¢ a series of equipment consisting of fixed thrust floats.

The pulling means can be, for example, metal or synthetic ropes connected to land winches or tugs or other naval means.

The means for supplying compressed air can be chosen, for example, from compressors or storage tanks connected to a pipe for carrying the air (common rail).

A further object of the present invention is the procedure for towing subsea pipelines carried out by means of pulling elements and the use of a series of equipment, connected to the pipeline itself, substantially containing floats, of which a fixed thrust part and a part with variable thrust or partially variable thrust which, placed in a predetermined position and at predetermined intervals, impart to the pipeline a graduated thrust according to the distance from the bottom in order to obtain a configuration of the `` festoon '' pipeline, i.e. with sections of suspended duct, in correspondence with the floats, alternating with supporting sections, characterized by the fact that each float, with variable or partially variable thrust, is substantially constituted by the equipment claimed in claims 1, 2 and / or 3.

The duct string typically consists of a series of standard length 12m (or multiples) pipe rods. Among its most important characteristics there is the so-called residual weight Wr - total weight of the immersed pipe not flooded - resulting from the difference between the weight in air of the various components (steel pipe, internal and external coatings, weighting in cement or gunite) and the hydrostatic thrust resulting from immersion in water. The residual linear weight wr expressed in unit of weight per unit of length is also used. In reality, the weight Wr (and wr) is nominal as there may be various uncertainties due both to the processing tolerances (steel thickness, coating thickness, weighting thickness, density of the various materials) and to various phenomena that are not well controllable (eg . soaking of gunite, accumulation of material on the pipe, etc.). These uncertainties, which can be expressed in terms of ∠† Wr (∠† wr), can be quite high in relative terms. This means that the thrust actually needed to reduce the value of Wr (wr) to an acceptable number for towing operations cannot be determined. Add to this that the same floats to be coupled to the tube contribute to the uncertainty because the thrust given by them is not deterministically defined due to the machining tolerances, etc. etc. To obviate these uncertainties and to ensure an effective reduction of the towing forces in safety, the floats are assembled with a sequence that sees a large quantity of passive floats mounted successively one to the other and at multiple spacing of the bars (one per bar to one ; one every two bars etc. as needed) interspersed with a few active floats to define a series of so-called festoons. The actual configuration of the floats (number of passive floats and distance between them; number of active floats) depends on the characteristics of the project (weight and string length, uncertainties, etc.).

The number of passive floats can preferably be from 2 to 20 times greater than that of active floats (the greater the uncertainties, the greater the number of active floats needed).

The number of passive floats is therefore determined on the basis of Wr. That of assets also from ∠† Wr. The thrust conferred by the set of active and passive floats is exuberant compared to what is necessary to allow a vertical “pulsating” movement of the pipeline which may tend to detach itself from the bottom, with a momentary reduction of friction and therefore of the necessary pull. Active floats ensure that the string does not actually stabilize and get out of control. The motion can be locally ascending with rapid return to the position of support at the bottom thanks to the intervention of the RV valve which acts as a sort of â € œsafety valveâ €. Moreover, thanks to the appropriately choked WP calibrated hole, the entire string has slow response times with respect to perturbations of the vertical equilibrium level, thus allowing the RV valve to act before the string has risen to an excessive level. As usual in these operations, the floats are connected to the piping in a removable way (eg through a metal band with snap release).

As far as the arrangement of the floats is concerned, it is preferable that those with variable thrust are placed near the beginning and end of each â € œfestestâ €.

Further information on the process can be found in the two applications WO2006 / 045357 and WO2008 / 017463 cited above, the content of which is incorporated in the present application by reference.

Example

An example of a towing operation includes the sequence:

â € ¢ (pre-towing): construction of the pipe string from a suitable pontoon by launching on the bottom (floats flooded by opening the VA). The floats are pre-installed during the construction of the line with the common air supply pipe already installed and connected to the floats (fixed to the pipeline). Abandonment of the string on the bottom with the floats connected and flooded, in a stable position

â € ¢ closure of the VA valves; connection of the tow rope to the head of the pipeline; connecting the air supply pipe to the compressors

â € ¢ introduction of air into the floats which gradually gain thrust

start of towing operations, which take place with the pipe that `` pulses '' in the vertical plane

at destination, release of the floats from the pipeline.

Claims (7)

CLAIMS 1. Equipment for towing subsea pipelines comprising: a float (G), with a prismatic shape with a horizontal or vertical axis, with variable or partially variable thrust, whose lower base is at least partially open through an opening or hole (WP) to operate as a â € œcampana dâ € ™ airâ €, having: at least one inlet (AP), consisting of a valve, through which air is supplied; at least one outlet (RV), consisting of a valve, through which the air is discharged; an actuator for controlling the valve to supply air; an actuator for controlling the valve to discharge air; means for connecting said apparatus to the pipeline; means for transferring air to said float; means for operating the actuator to control the valve (AP) to supply air according to a predefined level in the float; means for operating the actuator to control the valve (RV) to discharge air according to the distance of the float itself from the bottom. 2. Equipment as per claim 1 also having a bi-directional entry / exit control passage for the â € œtoo full ', consisting of at least one hole (TP). 3. Apparatus as per claim 1 or 2 also having a flooding passage for the float, bidirectional inlet / outlet, consisting of a valve (VA). 4. System for towing a subsea pipeline comprising: firing means connected to said duct by means of a firing head for firing the same; a series of equipment, connected to the pipeline itself, each consisting substantially of a float with variable or partially variable thrust whose lower base is at least partially open through an opening or hole (WP) to operate as a â € œ bell of airâ €, having: at least one inlet (AP), consisting of a valve, through which air is supplied; at least one outlet (RV), consisting of a valve, through which the air is discharged; a possible passage of control of the â € œtoo fullâ €, bidirectional inlet / outlet, consisting of a hole (TP); a possible flooding passage of the float, bidirectional inlet / outlet, consisting of a valve (VA); an actuator for controlling the valve for supplying air operated by means as a function of a predefined level in the float; an actuator for controlling the valve for discharging air operated by means according to the distance of the float itself from the bottom; means for supplying compressed air to the floats; a series of equipment consisting of fixed thrust floats whose lower base is at least partially open through an opening or hole (WP) to operate in an â € œair bellâ €. 5. Procedure for towing subsea pipelines carried out by means of pulling elements and the use of a series of equipment, connected to the pipeline itself, substantially containing floats, of which a fixed thrust part and a variable thrust or thrust part partially variable which, placed in a predetermined position and at predetermined intervals, impart to the pipeline a graduated thrust according to the distance from the bottom in order to obtain a configuration of the `` festoon '' pipeline, i.e. with suspended pipeline sections, in correspondence with the floats, alternating with supporting sections, characterized by the fact that each float, with variable or partially variable thrust, is substantially constituted by the equipment as per at least one of claims 1 to 3. 6. Process according to claim 5 where the number of fixed thrust floats is from 2 to 20 times greater than that of variable or partially variable thrust floats. 7. Procedure as per claim 5 where the variable thrust floats are placed near the beginning and end of each â € œfestestâ €.